Cast tool steel



Patented Dec. 8, 1953 Robert- D. Ahles, Schenectady,"=N. Y.,' assignor to -Genera'-1' Electric Company, -a-corporation of :New York NoDrawing. Application.trarch z'ails52,

, SerialI'No. 279,453

' :61Claims.. 01. 73-123) The present invention relates to a-cast tool steel. It is mcre particularly concernedwith a cast alloy steel which is particularly suitable for use inthe=manufacture of cutting tools, drawing dies and the like.

In general, cutting tools an'clthe like must possess high hardness, high resistance to the softeningeffect of-heat generated during their use, and high wear resistance. -At'thep'resent-time, the majority of cuttingtools are made from forgeable steels or alloy steels which, after working,

canbeheattreatedto obtain the required properties. While it has been propose'dWo-manuf-acture :such .toolsi'by ..a .casting technique, prior attempts in this direction have not been particularlysatisfaotory foriasnumber of reasons including the fact that the alloy steels .employed for.

casting cutting tools -have'had poor cutting life becauseof the chipping and'fai-ling of the cutting edge dueto the-massive-oarbide 'formation inthe micro structure of thecast steels. This structure is accompanied usually by low modulus rupture of such steels.

An object of the present invention is to provide a cast tool steel possessing a high hardness and high resistance to heat softening, plus a high wear resistance in combination with a high bending modulus of rupture. A further object of the invention is to provide cast cutting tools possessing physical characteristics comparable to those presently obtained by cold working and heat treating the forgeable tool steels.

Other objects and advantages of the invention will become apparent to those skilled in the art from the following description.

These advantages are obtained by providing a cast alloy free of the massive carbide structure typical to as-cast tool steels. The cast tool steel alloy of the present invention is particularly characterized by a high modulus of rupture and good hardening characteristics. The alloy is essentially composed, by Weight, of 1.5 to 2.25 per cent carbon, to per cent chromium, 4 to 6 per cent tungsten, 4 to 6 per cent molybdenum,

I 4 to 6 per cent cobalt, 4 to 6 per cent vanadium,

and from about 0.03 to 0.2 per cent magnesium, balance substantially all iron. For maximum modulus of rupture values, the chromium content is from 2 to 4 per cent and preferably about 3 per cent. Small amounts of nickel up to about 1 per cent may also be present. However, the nickel content should be kept as low as possible for best hardening characteristics. The alloy will also contain silicon in deoxidizing quantities of about 0.5 per cent. To obtain the desired magnesium content, it is usually necessary to add I 2 from-0. 25 to"0i'6 per-cent magnesium to-the melt. It will be noted that the alloy of the present invention differs from "the usual-cast tool-steel alloys in having a relatively high carbon content, a substantial content of otheralloying ele-' m'entsf and a small amount or magnesium as A preferred alloy within essential components. the scope of'the present invention is one containin'g,-by Weight, about 2 per cent'carbon, about 2 to-4 per cen'tchromium,'particularly aboutfi per cent chromium, about 5 percent of each of the elements molybdenum, tungsten, vanadium and cobalt, the designated amounts ofthe elements magnesium and silicon, with the'balance iron.

In'the manufacture of a tool such, for example, as a milling cutter, a-steel 'melted to the prescribed composition is -cast in a sand mold having the approximateconfiguration of the cutting-"tool. The cast product is annealedfor 8 to 10 hou-rs at'a temperature'of, for example, 750 C., and is thereafter ground or otherwise machined as required to obtain the necessary cutting edge or edges. The resultant product is hardened by a treatment consisting of heating at 900 C. for ten minutes and at 1250 C. for 15 minutes, oil quenching, tempering at 550 C. for three hours and finally air cooling from this temperature.

Test results on products of the present invention and a comparison thereof with the properties of other cast tool steels and forged tool steels have shown that the products of the prescut invention are much superior to the known cast alloys and are comparable in many respects with some of the better forged tool steel alloys. A cast high carbon-chrome tool steel containing from about 1.5 to 2.25 per cent by weight carbon, 12 per centchromium, and balance iron, after suitable heat treatment had a hardness on the Rockwell-C scale of 63 and a bending modulus of rupture of 69,000 p. s. 1. Another cast steel, containing about 1.35 per cent carbon, 0.83 per cent boron, 4.5 per cent chromium, 0.16 per cent molybdenum, 4.22 per cent tungsten, and a fraction of a per cent of vanadium, balance iron, had a Rockwell-C hardness of 64 and a modulus of rupture (bending) of 22,400 p. s. i. As compared with these cast steels, various samples of the cast steels within the scope of the present invention, after tempering, exhibited a Rockwell-C hardness of at least 60, and usually from 63 to 6'7, and a modulus of rupture (bending) of at least 190,000 p. s. i., generally from 200,000 to 240,000 p. s. 1.

These values for the cast alloy of the present invention compare favorably with those of the various well-known forged tool steel alloys exhibiting hardness values of from 60 to 70, and bending modulus of rupture of from 120,000 to about 400,000 p. s. 1. These values are much higher than those for a cast steel of the same analysis except that it contained no magnesium, which steel has a bending modulus of rupture of about 114,000 p. s. i. i

Photomicrographs of the magnesium-treated steel of the present invention show a structure characterized by very finely divided, evenlydispersed carbide particles both in the as-cast and heat-treated conditions. On the other hand, the magnesium-free steel of the same analysis had the typical as-cast massive carbide structure which cannot be broken up except by workmg.

A cast alloy steel of a one-half inch square cross section prepared in accordance with the present invention containing about 2 per cent carbon, 5 per cent each of chromium, tungsten, molybdenum, vanadium and cobalt, and 0.20 per cent magnesium, had an as-oast hardness of 64 on the Rockwell-C scale. After tempering at 550 C. for two hours, air cooling and re-tempering for one hour at the same temperature, the Rockwell-C hardness dropped to 60. When the cast part can be finished by a simple grinding operation, the necessary grinding can be carried out between these two tempering heat treatments. If the cast product is annealed after casting for the purpose of improving its machinability, it is thereafter heat treated for a total of about six minutes at 1235 C. and oil quenched, At this point, a sample had a Rockwell-C hardness averaging about 57. After a tempering treatment including 2 hours at 550 C., air cooling to a temperature below 90 F. and retempering at 550 C. for 1 hours, the Rockwell-C hardness 4 rose to 65. The hardened and tempered product had a modulus of rupture (bending) of 230,000 p. s. i.

As has been previously indicated, the chromium content of the alloy is preferably in the neighborhood of 3 per cent- It has been-found that those alloys containing about 2 to 3 per cent chromium have bending modulus of rupture values of about 240,000 p. s. i. while the alloys containing either no chromium or 5 per cent chromium have somewhat lower modulus of rupture values averaging about 220,000 p. s. i.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A cast steel alloy consisting essentially, by weight, of from 1.5 to 2.25 per cent carbon, about 0.03 to 0.2 per cent magnesium, from 0 to 5 per cent chromium, from 4 to 6 per cent of each of the elements tungsten, molybdenum, cobalt and vanadium, with the balance essentially iron.

1 2. A cast tool steel alloy consisting essentially, by weight, of about 2 per cent carbon, 2 to 4 per cent chromium, 5 per cent of each of the elements tungsten, molybdenum, cobalt and vanadium, 0.03 to 0.2 per cent magnesium, balance essentially iron.

3. A cast tool composed of the alloy of claim 1.

4. A cast tool composed of the alloy of claim 2.

5. A cast tool composed of the heat treated cast alloy of claim 1.

6. A cast tool composed of the heat treated cast alloy of claim 2.

ROBERT D. AHLES.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,147,121 Emmons Feb. 14, 1939 2,516,524 Millis et a1. July 25, 1950 

1. A CAST STEEL ALLOY CONSISTING ESSENTIALLY, BY WEIGHT, OF FROM 1.5 TO 2.25 PER CENT CARBON, ABOUT 0.03 TO 0.2 PER CENT MAGNESIUM, FROM 0 TO 5 PER CENT CHROMIUM, FROM 4 TO 6 PER CENT OF EACH OF THE ELEMENTS TUNGSTEN, MOLYBDENUM, COBALT AND VANADIUM, WITH THE BALANCE ESSENTIALLY IRON. 